Aliens. When you think of these you immediately think of two things - Star Trek-type rubber faces, or movie-type dark killing machines. But the ecology and personality of aliens are as diverse as the worlds they herald from. Here are a few pointers on how to give your aliens a little more depth than having them look like a man with pointed ears saying 'what is this human emotion called love?'
For ease of use, many aliens have a humanoid form, which is handy in the great scheme of things. You don't have to worry how they'll interact with the technology and setting.
But they don't have to be like that. Environment and location dictate the appearance of a living being, not the make-up artist.
Take a few examples from our own world. In the deserts of the equator, creatures have developed a metabolism and appearance that protect them from the searing sun. In the depths of the ocean, fish have developed a physique that helps them glide through the water and breathe its limited oxygen. Birds have developed their form and abilities to exist in the most inaccessible places of the world. Even the human race walks on two legs, which reflects their origins in the tall grasses of the African plains. The world the alien heralds from should reflect their physique.
So, for example, let's say that the planet is a desert world with very few locations where there is water. A single great ocean surrounds the planet's equator like a belt. The aliens would have built their civilisation about this water. Physically, they may be insectoid, with huge carapaces that bend over their heads to protect them from the searing sun. They may be long-legged for ease of moving over the dunes, and have large sack-like growths on their backs to store water, like a camel. Their eyes have multiple eyelids to protect from the UV glare, and they have tendrils over their mouths and nose to protect them from the sand storms. Alternatively, the aliens could be lizards, piscine, bird-like, or invertebrates. They could even be huge gas- filled floating jellyfish!
So, there's the first phase of the creation process: Environment equals physical appearance.
In many respects, people think that to reflect an alien language the aliens just speak differently, as different as English from Japanese, or Russian from Spanish. But this does not need to be the case.
Many creatures on this earth communicate in different ways. Insects use both touch and scent, mammals use growls and calls (like dolphins), birds use a variety of whistles and hoots. Some creatures even use colour to communicate their intentions.
So this could translate into the alien world. How about if the aliens didn't communicate through speech but through a series of clicks and whistles at different pitches. Or they communicate through sign language. They could even be telepathic. This will make them exceptionally different.
This works well on different levels. If the players encounter them for the first time, talking will be difficult, and will make for an excellent roleplaying opportunity.
As mentioned before, the world the alien heralds from may dictate their appearance, but how will that affect their interaction with other species? Perhaps the alien needs to be segregated from other species and kept in a room where the atmosphere and pressure suits their biological makeup. Perhaps they have to wear environment suits to traverse other places. Perhaps they simply need a face mask so that they get a quota of gases that can only be found on their own world.
Aliens that walk, talk, and interact normally in any environment are just men with strange appearances. Limiting, or even increasing, their abilities and function due to their biology adds an extra dimension.
Not all aliens have to be a race of super-beings, far beyond the capabilities of the human race. They also don't have to be evil two-dimensional killing machines either.
Intelligence has a large bearing on the function of the alien. A creature of bestial intelligence cannot be considered evil, it is simply doing what it must do to survive or procreate. So, when you land on that planet and a bunch of razor-sharp ripperlizards come bounding out of the purple trees, they don't want to kill you because they're evil, but because it is in their nature to do so.
It's a simple matter of discerning two things: their diet and their timidity. A vegetarian creature of a timid nature will not be much of a threat to the PCs, but then a vegetarian with an aggressive nature might be. The same goes for meat-eaters.
Intelligence in an alien should not dictate their attitude and feelings towards outsiders. Higher intelligence does not necessarily mean infallibility or greater moral standing. The aliens will have several different levels of intelligence, ranging from the neanderthal to the super- brain, but this doesn't reflect their morals.
Take the Roman Empire, for example. They were the most civilised, artistic, and prolific race of the time in ancient world. Their Empire is the basis of modern society. Their mathematics and architecture outshone their neighbours' yet they still thought it perfectly alright to watch men slaughtering each other in an arena. And they found nothing wrong with it. Because they were greater and (allegedly) smarter, they thought this allowed them to do such things.
Intelligence will also affect communication with other species. Lesser intelligent aliens would have little to share or offer, whereas higher intelligence species may have plenty to talk about and discuss. Where species connect on an intellectual level may help determine the outcome of relations.
Morals And Attitude
Talking of morals, this is something that will make the alien far different from other species. They may see violence and death as a natural order and actively seek out species to kill. They may decide that all other species are greater than them and worship them as gods, or that other species are lesser beings and need to be exterminated. As far as the alien is concerned, their morals and attitudes regarding themselves, existence, and other species is completely justified. It is not just the views of individual aliens you must take into account, but the entire continent or world.
So, the aliens may revere life, or hate it, or are indifferent about it. They may have religious overtones or a completely different theory on evolution to suit their existence. Although there is always room for a little variance on the individual aliens, the broader belief system or attitude must be considered as a basic layout for the personality.
What are the aliens capable of with the knowledge and intelligence that they have? Do they exist in a permanent middle-age society or have they unlocked the secret to interstellar travel? When encountering new races, the PCs will be confronted by not only the sheer difference of an alien but also what help or hindrance they present. If they land on a medieval world and are treated as the enemy, then they won't be under much threat from bows and arrows as they take off in their starship.
Alternatively, if the aliens have nuclear power then getting whacked by a missile may cause more than a few problems.
Technology need not be limited to the physical boundaries of our own world. The technology of the aliens may be quite, quite different. What if they grew their technology, flew the spacelanes in huge creatures bred for spaceflight? They may even want to use the PCs as raw material! If the aliens have a greater technology to our own, they could be a great help to the future of mankind or possibly a great threat. If they have lesser technology than ours then perhaps mankind could help them grow and increase in ability, or perhaps not...
On a personal note, when I began sci-fi roleplaying I developed something that I called the 'Theory of Mirrored Evolution' that helped me through my first games. I didn't have to worry about the ecology of the aliens. I just assumed that because the Earth was created due to a galactic chance from the same star stuff that other suns are made of, then why couldn't the other worlds be similar to our own, with differences noticeable enough to make them alien? It was a simple matter then to utilise humanoids with different features and attitudes. This took the work out of alien design so that I could concentrate on the game and get used to the setting. Nowadays, I use the above guidelines and the games have more depth because of it.